The bulletproof performance as an index for ascertaining the quality of a bulletproof fabric determines how light weight of the fabric is sufficient to prevent the penetration of bullets. It is well known that this performance greatly depends upon the tensile strength and initial tensile modulus of original fibers used. The term "tensile strength" as used herein refers to the quotient of the tensile tenacity of a material divided by the weight of the material.
As a matter of course, preferred bulletproof fabrics have higher bulletproof performance. Since bulletproof fabrics are continuously kept as an article of clothing on the body of a wearer for many hours, they should not lower the mobility of the wearer and they are further required to have light weight. When bulletproof fabrics are made into articles of clothing or other products, they are usually used in layers.
The conventional bulletproof fabrics known in the art include those made of aramid fibers (hereinafter referred to as conventional example 1). Aramid fibers are, however, not suitable for this purpose because of their insufficient tensile strength and initial tensile modulus. In addition, the wounding and killing power of small arms has remarkably increased in recent years. For these reasons, there has been a great demand for higher performance bulletproof fabrics as a countermeasure against these small arms.
This problem may be solved, for example, by increasing the number of layers of the bulletproof fabrics made of aramid fibers. In this case, however, it is attended with a significant increase in weight, and taking into consideration the fact that the bulletproof fabrics are used for the articles of clothing, it results in a fatal defect depending upon the level of weight increase.
By the way, since bulletproof fabrics are literally fabrics, warps and wefts are meandering or winding by their intersection. For example, when a warp intersects with a weft so as to pass under the weft, the warp intersects with the next wefts so as to pass over these next wefts. Thus, the warps and wefts are meandering through each other. It follows that each of the yarn-forming filaments is also undulating or winding.
FIG. 3A is an enlarged schematic view in part of undulating filament 10, showing that force is exerted to filament 10 in the direction of arrow A. In such undulating filament 10, the force exerted to the outer side of the undulating portion becomes unbalanced with and is greater than the force exerted to the inner side of the undulating portion.
FIG. 3B is an enlarged schematic view in part of liner filament 20, showing that force is exerted to filament 20 in the direction of arrow A. In such linear filament 20, the force is uniformly exerted all over the portions of each filament.
As can be seen from the comparison between FIGS. 3A and 3B, filament 10 given unbalanced force as shown in FIG. 3A can only resist weaker force as compared with filament 20 given well-balanced force as shown FIG. 3B, so that the tensile strength of the fabric is decreased. In the case of high-tenacity fibers such as particularly used for the bulletproof fabrics, their elongation at break is small and their knot tenacity is extremely lower than their linear tenacity (i.e., they have a small retention of tenacity). Therefore, when each filament of the high-tenacity fibers is undulating, their original high tenacity turns to an extremely lower one.
In order to ensure that the exertion of force does not become unbalanced and the mechanical properties of each filament, such as tensile strength, can therefore be exhibited to the full extent, it would be necessary to increase the linearity of the filament.
When twisted yarns are used in the bulletproof fabric, each of the filaments forming a yarn is made by the twist of the yam to undulate or wind in this yarn, for which same reason the mechanical properties of each filament cannot be fully exhibited. For example, the tenacity of a twisted yarn decreases several percent to several tens percent as compared with the linear tenacity of an original filament.
In addition, twisted yarns have a tendency to take a round section at a higher twist coefficient, so that the diameter of twisted yarns (i.e., yarn height in the direction of fabric thickness) increases as compared with flat yarns and the meandering of the twisted yarns becomes marked when they are made into a fabric.
The linearity of filaments may be retained by the use of non-twisted yarns. In the case where non-twisted yarns are used in the actual operation, however, there arises a serious problem that the breaking or fibrillation of yarn-forming filaments is liable to occur by the warps' own abrasion or contact in the weaving. From a viewpoint of avoiding such a problem and thereby improving the weaving efficiency, there is no other way in most cases but to use twisted yarns in the weaving under the existing circumstances.
For the weaving density of a fabric, the meandering of fabric-forming yarns becomes sharper at higher weaving density, so that yarn-forming filaments cause undulation. Therefore, the tensile strength of the filaments cannot be exhibited to such an extent as expected in theory, and it will be decreased in practical use.
Furthermore, in the case of a fabric with higher weaving density, mutual binding force at the points of intersection between the warps and the wefts becomes stronger, and when bullets or other small projectiles collide with the fabric, shock waves are reflected and accumulated in the form of stress at these points of intersection. If the mutual binding force at the points of intersection is weak, shock waves are transmitted through the fabric-forming fibers to cause a dispersion of energy over a wide area, so that the fabric can withstand the shock of the bullets or other small projectiles. The accumulation of stress derived from the shock waves as described above, however, causes breaking of the fabric.
If there is the meandering of yarns in a fabric, when bullets or other small projectiles hit the fabric, the fabric-forming yarns are first drawn by the bullets or other small projectiles to make their winding or meandering sharpened at a limited part, and the yarn-forming fibers are then drawn. In other words, the fibers are not drawn in the direction of their own axis until the winding becomes sharper at a limited part, and the fibers are given strong compressive force in a direction perpendicular to the fiber axis. Therefore, when the deformation of a bulletproof fabric caused by the invasion of a bullet is too slow to follow the speed of the bullet, the fabric-forming yarns only undergo compressive breaking and the kinetic energy of the bullet is not effectively converted into another energy necessary for the fiber breaking, so that the fabric is readily perforated.
The linearity of filaments may be improved by decreasing the weaving density to temper the meandering of yarns. In this case, however, when a fabric catches bullets or other small projectiles, the weave pattern of the fabric deviates from its original pattern, so that the fabric is readily perforated to form holes and the bullets or other small projectiles can pass though these holes. Therefore, bulletproof fabrics should be produced with increased weaving density, so that the mechanical properties of filaments cannot be fully exhibited.
As the material fiber, there have been proposed ultrahigh molecular weight polyethylene fibers (hereinafter sometimes referred to UHMW-PE fibers) having specific strength and specific modulus both exceeding those of aramid fibers. UD fiber-layered sheets made of UHMW-PE fibers and binder resins (generally called "shield materials"; conventional example 2.1), and fabrics made of UHMW-PE fibers (conventional example 2.2) have begun to be put to practical use.
However, conventional example 2.1, although it is effective against a certain kind of special ball cartridges, is not suitable to cope with the threats of relatively low level, such as shrapnels (i.e., shell fragments). In addition, it requires a binder resin having no direct relation to the energy absorption, and such a resin is contained to the amount of 30 wt % or higher, resulting in a defect that the fabric becomes too heavy.
In the case of conventional example 2.2, as described above, the mechanical properties of filaments (i.e., UHMW-PE fibers) cannot be fully utilized because each filament in the fabric is undulating. As a result, the bulletproof performance of the fabric cannot be exhibited to an extent as expected.
JP-A 8-502555/1996 discloses a bulletproof fabric proposed in an attempt to solve the above problem by giving a twist to the filaments (hereinafter referred to as conventional example 3). Conventional example 3 is a bulletproof fabric comprising a multifilament yam made of high-strength filaments each having a tenacity of about 7 g/d or more and a tensile modulus of about 150 g/d or more, and each requiring an energy for breaking of about 8 J/d or more. The multifilament yarn has a portion in which the filaments are entangled with each other and a portion in which the filaments are arranged substantially in parallel to the lengthwise direction. In the portion of filaments arranged in parallel, the mechanical properties of filaments can be exhibited to absorb impact energy and the flatness of these filaments can give a close weave, thereby making an attempt to improve the bulletproofness. In the portion of entangled filaments, they are bound with each other to prevent the breaking or fibrillation of filaments at the time of weaving, thereby making an attempt to prevent a deterioration in weaving efficiency.
However, conventional example 3 can only exhibit the mechanical properties of filaments on a low level in the portion of entangled filaments because of their non-linearity. The presence of such a weak portion, even if in part, is responsible for the initial breaking of filaments in the weak portion when external force is exerted thereto, so that the overall strength of the fabric is lowered. Therefore, even in the conventional example 3, the mechanical properties of filaments are not yet fully utilized.